The present invention relates to an arrangement for measuring antenna signal strength, the arrangement comprising:
a first amplifier section for receiving an antenna signal for providing a first output signal, and having a first automatic gain control stage with a first attenuation factor controlled by a first automatic gain control signal;
a second amplifier section for receiving a first input signal, derived from said first output signal, for providing a second output signal, and having a second automatic gain control stage with a second attenuation factor controlled by a second automatic gain control signal;
automatic gain control means arranged to receive a second input signal derived from the second output signal and to generate said first and second automatic gain control signals in dependence on the second output signal;
the arrangement being arranged such that:
for values of said antenna signal below a predetermined threshold level, only said second automatic gain control stage may be active with increasing second attenuation factor with increasing values of the antenna signal; and
for values of said antenna signal above said predetermined threshold level, said first automatic gain control stage is active with increasing first attenuation factor with increasing values of the antenna signal.
Such an arrangement is known from U.S. Pat. No. 4,403,346. The arrangement known from this prior art document will be illustrated with reference to FIG. 1 which is also presented as FIG. 7 in that document.
FIG. 1 shows an automatic gain control (AGC) circuit for a wide band tuner as used in television systems. An antenna (not shown) receives a signal and generates an output signal ANT INPUT applied to a first AGC stage 2. The output of the first AGC stage 2 is sent to an UHF/VHF RF circuit 40. Typically, the UHF/VHF RF circuit will comprise three parallel circuits, two for two different VHF frequency bands and one for one UHF frequency band. Instead of three different bands, one band may also be applied depending on the application concerned.
The output of the RF circuit is connected to the input of an UHF/VHF selection switch 7. The selection switch comprises an U/V switch 7a and a second AGC stage 7B in series. The output of the selection switch 7 is fed to two superheterodyne sections which are not shown in FIG. 1. Superheterodyne sections and the way they operate are known to persons skilled in the art and need no further explanation here. The output of the second superheterodyne section is a demodulated picture signal. Moreover, the second superheterodyne section generates an AGC control voltage VAGC which is an input to an AGC signal source 50.
The AGC signal source 50 comprises a differential amplifier A50 and an inverting amplifier A52. The input of the inverting amplifier A52 is connected to the inverting input of the differential amplifier A50. The non-inverting input of the differential amplifier A50 is connected to a node E50 which is connected to the AGC control voltage VAGC through a resistor R50. Node E50 is also connected to the input of inverting amplifier A52 and to the anode of a diode D50. The cathode of diode D50 is connected to a slider of a trimmer resistor VR50, one side of which is connected to a power supply voltage and the other side of which is connected to ground.
The connection point between resistor R52 and the inverting amplifier A52 is called E52 and is also connected to the anode of a diode D52. The cathode of the diode D52 is connected to a slider of a trimmer resistor VR52, one side of which is connected to the power supply voltage and the other side of which is connected to ground.
When the voltage on node E50 is above a threshold voltage VTH1, diode D50 will conduct. Below this value, diode D50 will be blocked. When the voltage on node E52 is above a threshold voltage VTH2, diode D52 will conduct. Below this value, diode D52 will be blocked. The trimmer resistors are set such that VTH1 greater than VTH2.
The output voltage of differential amplifier A50 is used as AGC control signal VAGC1 for the first AGC stage 2. The output voltage of inverting amplifier A52 is used as AGC control signal V28 for the second AGC stage 7B.
Now, the operation of the arrangement of FIG. 1 will be explained with reference to FIGS. 2 and 3 which are identical to FIGS. 8 and 9, respectively, of U.S. Pat. No. 4,403,346. As shown in FIG. 2, as long as VAGC is below VTH2, neither diode D50 nor diode D52 is conducting. Then, VAGC2 (input voltage of inverting amplifier A52) shows a linear relationship with VAGC. Since almost no voltage difference will be present between the differential inputs of differential amplifier A50, VAGC1 will be substantially equal to 0 V.
When VAGC exceeds threshold voltage VTH2, diode D52 starts conducting and the input voltage VAGC2 of inverting amplifier A52 remains fixed at a predetermined level P2. However, from that moment onwards, a current flows through resistor R52 and a voltage difference will be built up between the inputs of differential amplifier A50. Then, VAGC1 shows a linear relationship with VAGC.
However, upon VAGC exceeding threshold level VTH1, diode D50 starts conducting, resulting in node E50 remaining at a fixed voltage. Therefore, for VAGC values above VTH1, the value of VAGC1 remains fixed at a predetermined level P1.
FIG. 3 shows the total gain of the tuner including components 2, 40, 7 in dependence on the value of control voltage VAGC. The first AGC stage 2 and the second AGC stage 7B are both implemented in such a way that their respective gains are at maximum when their AGC control voltages VAGC1 and VAGC2, respectively, equal 0 V and their gains can only decrease with increasing values of AGC control voltages VAGC1 and VAGC2, respectively.
As long as VAGC is below a predetermined threshold value VTH0 (VTH0 less than VTH2), both AGC stages 2, 7B show maximum gains and the total tuner gain is G0. When VAGC exceeds threshold voltage VTH0, the second AGC stage 7B starts to show a linearly decreasing gain relation with increasing values of VAGC until VAGC reaches threshold VTH2. From that moment onwards, VAGC2 remains fixed and the attenuation factor of the second AGC stage 7B remains fixed. Moreover, from that moment onwards, VAGC1 starts to show an increasing linear relation with VAGC (FIG. 2) and the first AGC stage 2 starts to show a linearly decreasing gain relation with increasing VAGC. When VAGC exceeds threshold VTH1 also control voltage VAGC1 remains fixed (see FIG. 2). So, for values of VAGC above VTH1 both AGC stages 2, 7B remain at a fixed attenuation level. The total tuner gain will then remain fixed at a predetermined value of G1.
Since the attenuation of the first AGC stage 2 only starts after the attenuation of the second AGC stage 7B has reached its maximum, the concept of AGC shown in FIGS. 1, 2 and 3 is also known as xe2x80x9cdelayed AGCxe2x80x9d.
As described above, the signal VAGC is generated by the superheterodyne section (not shown) that outputs the demodulated picture signal. Thus, essentially, the first and second AGC stages 2, 7B in the prior art according to FIG. 1 are controlled by the output of the arrangement itself. In practice, the value of threshold VTH0 (FIG. 3) is determined by a reference voltage with which the demodulated picture signal is to be compared. The thresholds VTH1 and VTH2 are determined by the settings of the sliders of variable resistors VR50 and VR52. These settings are made in the tuner factory and will, normally, not be accessible to buyers of the tuners.
The object of the present invention is to provide a tuner arrangement arranged to measure and possibly display the strength of the antenna signal supplied to the tuner arrangement.
This object is obtained by an arrangement as defined in the opening paragraph, wherein a processor is provided which is arranged to calculate said antenna signal strength in accordance with the following steps:
(a) establishing whether or not said first automatic gain control stage is active;
(b) if said first automatic gain control stage is active, increasing said predetermined threshold level to a first new threshold level such that this first new threshold level is equal to said antenna signal strength and the first automatic gain control stage starts to become inactive, and calculating said antenna signal strength as being equal to said first new threshold level;
(c) if said first automatic gain control stage is inactive, decreasing said predetermined threshold level to a second new threshold level such that this second new threshold level is equal to said antenna signal strength and the first automatic gain control stage starts to become active, and calculating said antenna signal strength as being equal to said second new threshold level.
With such an arrangement, the antenna signal strength can be measured which can be used to align an (outdoor) antenna connected to the arrangement to maximum signal strength reception. This is especially advantageous in those areas where outdoor antennas are used, such as in developing countries.
Such an arrangement can also be used to select a channel having the strongest signal strength of a certain transmitted program. Moreover, signal strengths of channels adjacent to an actual channel can be measured in this way. Then, VTH2 can be varied to counteract intermodulation from the adjacent channels and to obtain an optimum signal-to-noise ratio based on channel frequency and signal strength of adjacent channels.
In an alternative embodiment, the present invention relates to an arrangement for measuring an antenna signal strength, the arrangement comprising:
a first amplifier section for receiving an antenna signal, for providing a first output signal, and having a first automatic gain control stage with a first attenuation factor controlled by a first automatic gain control signal;
a second amplifier section for receiving a first input signal derived from said first output signal, for providing a second output signal and being provided with a second automatic gain control stage with a second attenuation factor controlled by a second automatic gain control signal;
automatic gain control means arranged to receive a second input signal derived from the second output signal and to generate said first and second automatic gain control signals in dependence on the second output signal;
the arrangement being arranged such that:
for values of said antenna signal below a predetermined threshold level only said second automatic gain control stage may be active with increasing second attenuation factor with increasing values of the antenna signal;
for values of said antenna signal above said predetermined threshold level said first automatic gain control stage is active with increasing first attenuation factor with increasing values of the antenna signal;
wherein a processor is arranged to calculate said antenna signal strength in accordance with the following steps:
(a) establishing whether or not said first automatic gain control stage is active;
(b) if said first automatic gain control stage is active, increasing said predetermined threshold level to a first new threshold level such that this first new threshold level is equal to said antenna signal strength and the first automatic gain control stage starts to become inactive, and calculating said antenna signal strength as being equal to said first new threshold level; if said predetermined threshold level cannot be increased to said first new threshold level but only to a predetermined maximum level, then, calculating said antenna signal strength as being larger than said predetermined maximum level;
(c) if said first automatic gain control stage is inactive, decreasing said predetermined threshold level to a second new threshold level such that this second new threshold level is equal to said antenna signal strength and the first automatic gain control stage starts to become active, and calculating said antenna signal strength as being equal to said second new threshold level; if said predetermined threshold level cannot be decreased to said second new threshold level but only to a predetermined minimum level, then, calculating said antenna signal strength as being smaller than said predetermined minimum level.
This alternative embodiment has, essentially, the same functionality as the first mentioned embodiment. However, here it is taken into account that shifting of the predetermined threshold towards the actual value of the antenna signal strength may actually not be possible, and only either an upper limit or lower limit of the actual antenna signal strength can be calculated.
Further embodiments of the arrangement according to the invention are claimed in dependent claims.
Moreover, the invention relates to methods of measuring antenna signal strength. The invention also relates to computer program products having instructions for carrying out essential steps of the invention, and to computer readable mediums provided with such computer program products.